1. Field of the Invention
The present invention relates to a device and a method to determine the basis weight of a moving material web, in particular a fibrous web, including at least one microwave sensor which has an element for coupling the microwaves and a reference element, whereby the coupling element and the reference element are located at a distance from each other in a manner that the material web can be moved between them.
2. Description of the Related Art
Basis weight is generally understood to be the total weight of, for example a fibrous web, in other words the weight of all fibrous web components such as fibers, ash and water together. The fibrous web may be a paper web, tissue web or cardboard web.
Previous measurement technologies are usually based on the absorption of health hazardous radioactive radiation, so that such a measurement is always connected with considerable regulatory constraints.
The fundamental measuring principle is already known for example, from patent specification U.S. Pat. No. 4,755,678 by J. R. Izatt et. al. “Simultaneous measurement of moisture content and basis weight of paper sheet with sub-millimeter laser”, or from the publication by E. Nyfors and P. Vainikainen “Industrial Microwave Sensors”, published by Artech House Norwood 1989. One characteristic of such measurements is that the measurement signal is subjected to extreme intrinsic distance sensitivity.
Patent application EP 1 703 275A1 discloses a measuring device and a measuring method, and describes a microwave measurement which indeed considers this fact by measuring the distance and an appropriate signal correction, whereby the thus obtainable measurement results are too imprecise in practice.
What is needed in the art is a device and a method to determine the basis weight of web materials, which allows for a more precise basis weight measurement on a moving material web, using microwave radiation.